1. Field of the Invention
This invention relates to a radiation detector assembly containing a photo-cathode for emission of photo-electrons upon impingement of light photons. In particular, this invention relates to a radiation detection assembly containing an electron multiplier such as a multichannel plate for multiplying photo-electrons emitted from a photo-cathode. Still more particularly, this invention relates to a radiation detective assembly for a two-dimensional imaging device applicable in nuclear medicine.
2. Description of the Prior Art
Radiation detecting devices utilizing a planar scintillation crystal and a plurality of photomultiplier tubes in an optical scanning arrangement with respect to the crystal are currently in use in a number of applications, such as in medical diagnostic equipment. A particular form of such a radiation detecting device is described in U.S. Pat. No. 3,011,057 (Anger), which discusses a radiation detecting device of a type in wide commercial use for locating a radiation source in two-dimensional coordinates. Such a device is known as a scintillation camera or gamma camera, and it is widely applied to rapidly project an image of the distribution of radioactivity throughout an object under examination, such as an organ of a living subject that has ingested a diagnostic quantity of a radioactive isotope.
A scintillation camera of this type produces an image of the radioactivity distribution by detecting individual gamma rays emitted from the distributed radioactive isotope. The gamma rays pass through a collimator to produce scintillations, that is scintillation light of a certain wave-length, in a thin planar scintillation crystal. The scintillation crystal or scintillator is typically made of thallium activated sodium iodide and has the form of a thin disc. A light guide or light pipe is provided for passing on the scintillation light. The scintillations are detected by an array of individual photomultiplier tubes which view overlapping areas of the crystal. Appropriate electronic circuits translate the outputs of the individual photomultiplier tubes into X and Y coordinate signals and a Z signal which indicates generally the energy of the scintillation event and whether that energy falls within a selected energy window. A visual display of the radioactivity distribution in the object may be obtained by coupling the X, Y and Z signals to a cathode ray oscilloscope or other image display device, where the individual scintillation events are displayed as small spots of light positioned in accordance with the X and Y coordinate signals.
Further details of a scintillation camera are described in U.S. Pat. No. 3,683,185 (Muehllehner).
Efforts have been made to reduce the weight and the size of radiation detecting devices which are determined to generate two-dimensional images of an object. This is particularly true for X-ray detection devices (see i.e. U.S. Pat. Nos. 4,104,516 and 4,140,900). These devices generally contain a vacuum window provided in an envelope, a scintillator, a photo-cathode attached thereto, and an imaging layer or screen spaced from the photo-cathode. The imaging layer or screen is directly observed. Means for providing an electrical signal in accordance with the displayed image are not provided.
Improvements on the Anger type gamma ray camera have focused on increasing the spatial resolution capabilities and increasing the capability for distinguishing between incident and scattered radiation, for instance, by incorporating a gamma ray sensitive, proximity type image intensifier tube (see i.e. U.S. Pat. No. 4,221,967). Yet, such a design requires an additional conversion stage where the scintillation light is converted into photo-electrons and subsequently again into light. It is desirable to avoid such an intermediate step. It is also desirable to change the camera design such that a large number of individual photomultiplier tubes or solid state photo-detectors for detecting the interaction sites of impinging gamma rays are no longer required.